Matt, the DNC doesn’t have to explain anything. They are a private outfit, and they have no responsibility to anyone.
They could, if they wish, put out a press release tomorrow that states “Sanders cannot have our nomination, no matter how many votes he gets.” And that would be perfectly legal.
HOWEVER, doing that would alienate way too many people, as well as create an independent candidacy that would doubtless result in the re-election of Trump. So, the game must be played — — Sanders must lose “fair and square” (in the eyes of the public) so the Dem nominee retains party support and Sanders is put in a box which he cannot emerge from without ending up being the Sore Loser who caused Trump to be re-elected.
Now, why are they doing that? Why would it be so horrible if Sanders were nominated, one asks?
This is where Sanders’ strength — grassroots donations — turns into his liability. The money people in the party want Anybody But Bernie. And those Money People control a HELL of a lot more money than the grassroots donors do, when it comes to the period between the convention and the general.
Nominate Bernie, and he runs out of financial oxygen, as the big donors stay home and the little donors can’t supply enough ammunition.
Notice that nothing about what I wrote had squat to do with policy. It’s money.